Coated flash lamp and manufacture thereof



May 7, 1957 R. M. ANDERSON 2,791,113

COATED FLASH LAMP AND MANUFACTURE THEREOF Filed April 7, 1954 w M, 4 I,1

Inventor:

Robert M. Anderson,

His Attorney COATED FLASH LAIWP AND MANUFACTURE THEREOF Robert M.Anderson, Euclid, Ohio, assignor to General Electric Company, acorporation of New York Application April 7, 1954, Serial No. 421,571

6 Claims. (Cl. 6731) My invention relates to flash lamps in general andto the manufacture thereof, and particularly to flash lamps of the typecomprising a bulb or envelope of glass or similar material andcontaining a charge of readily combustible light-giving material andcombustion-supporting means which, on ignition, burns with greatrapidity to produce a momentary flash of actinic light, and more particularly to light-transmitting protective lacquer coatings for saidlamps.

In flash lamps of the above-indicated type, the bulb is customarilyprovided with an interior, light-transmit ting, protective coating of asuitable material, such as a lacquer for instance, in order tostrengthen the bulb and prevent the cracking thereof, upon chargeflashing, by hot burning particles of the combustible material. Asimilar coating is applied to the exterior surface of the bulb toprovide additional protection against explosion and flyingglassparticles.

It is desirable in ordinary flash lamps, and virtually necessary inminiature flash lamps, particularly those having a butt seal, toeliminate the interior coating. For example, omitting the interiorcoating in ordinary flash lamps such as that commercially designated asNo. 5, and similar lamps, will effect a reduction in manufacturing costssince the expense of spraying the bulb interior and drying the lacquercoating is eliminated. In sub-miniature flash bulbs of very small volume(about 6 cc. for example), such as disclosed and claimed in co-pendingU. S. application Serial No. 421,510, Anderson et al., filed of evendate herewith and assigned to the assignee of the present invention, theneck opening is usually too small to admit a lacquer spraying nozzleand, although the lacquer may be poured into the bulb, it is impossibleto drain a liquid-filled bulb of the size contemplated due to the smallopening and the surface tension of the liquid. Even water will not drainfrom such a miniature lamp unless an opening is made in the bulb otherthan the neck opening.

When the interiorprotective lacquer coating is omitted for any oftheabove reasons, several problems arise. The outside lacquer remaining maynot be sufficiently strong to contain the forces generated when the lampis flashed, thus increasing the possibility of explosion of the lamp andinjury to the user. Applying a thicker outside lacquer film to increasestrength does not solve the problem since lacquers of the typecustomarily used, such as cellulose acetate, exhibit considerablesmoking, charring and even burning when a lamp is flashed, thusdrastically reducing tensile strength of the film. Charring, burning orany heat discoloration of the outer lacquer film also reduces theefiective light output of the lamp. Increased film thickness is alsoobjectionable from the tive coating ofsuflicient strength to Withstandthe forces generated on charge-flashing of the lamp in the absence ofany coating on the interior bulb surface.

Another object of my invention is to provide a bulb for a photoflashlamp or similar device provided with an exterior coating capable ofabsorbing the thermal shock produced on combustion of the charge withlittle or no smoking, charring or burning of the coating in the ab-.ieliCe of any protective film on the interior surface of the Stillanother object is to provide an exterior composite protective coatingfor flash lamps including one component which softens and adheres to theglass at the temperature produced on charge-flashing of the lamp, tocontain glass fragments should shattering of the bulb occur, and asecond component which retains adequate tensile strength to minimize thepossibility of an explo- Briefly stated, the above objects may beaccomplished, in the absence of any interior coating, when the exteriorglass bulb surface is provided with a double-layer coating consisting oftwo distinct superimposed lacquer films possessing different propertiesas hereinafter decribed. A lacquer film, hereinafter called the buflerlacquer or layer, is applied directly to the exterior bulb surface anddried. The buffer layer is of such composition that it will resistthermal shock, absorbing the heat generated on flashing of the lamp,thus insulating the outer lacquer from intense heat, and has arelatively low softening point becoming soft and tacky at thetemperatures present so as to adhere to the glass and prevent rapiddispersion of glass particles should an explosion take place. After thebuffer layer has dried, a second lacquer layer is applied. This secondor outer lacquer is composed of a suitable material such that the driedfilm possesses a tensile strength of about 8000 to 10,000 p. s. i. andexhibits a tendency to retain some effective strength during the shortinterval of high temperature (at least 300 C.) encountered oncharge-flashing of the lamp, thus provid ing the strength necessary tocontain effectively the internal pressures generated. In addition, theouter layer should have some toughness, that is, abrasion and tearresistance, and should not yellow or discolor unduly with age.

Thus it can be seen that a bulb exteriorly coated with thedouble-lacquer films possessing the above-mentioned propertiesadequately surmounts the problems arising upon omission of an interiorlacquer film.

Further objects and advantages of my invention will appear from thefollowing description of species thereof and from the accompanyingdrawing in Which:

Fig. 1 is an elevation of a sub-midget flash lamp of buttseal type whichmay be coated in accordance with my invention;

Fig. 2 is an elevation of a flash lamp of conventional stem seal typewhich may also be coated in accordance with the invention; and

Fig. 3 is a fragmentary sectional view of a flash lamp bulb illustratingthe application thereto of a coating in accordance with my invention.

Referring to the drawing, Fig. 1 illustrates a submidget flash lamp ofthe type generally known as a butt-seal lamp comprising a bulb orenvelope 1 of glass or other suitable material having positioned thereina mount comprising a pair of lead-in wires 2, 2 tied to gether by aglass head 4, said lead-in wires connected at their inner ends to theopposite ends of a filament 3 of tungsten wire, the outer ends of saidleads 2, 2 being connected respectively to the shell contact 7 andcenter contact 6 of the base 5. The inner ends or tips of the lead-inwires 2, 2 are provided with small beads or coatings 9, of a suitableprimer or fulminating material such as, for example, a mixture ofzirconium, magnesium and potassium perchlorate powders bonded togetherby a suitable binder such as nitrocellulose. The primer beads 9, 9 arelocated on the lead-in wires 2, 2 at the region of the connection of thelatter to the filament 3 so as to embed at least the end portions ofsaid filament and thus become ignited thereby upon passage of anelectric current through the filament.

Disposed within the bulb 1 and around the filament 3 and primer beads 9,9 so as to be Within effective lgllltion range thereof, is a charge ofreadily combustible light-giving material 10, such as leaf foil, wire,shredded foil, or ribbon which, on ignition by the igniter means 3, 9,9, burns with great rapidity to produce a momentary flash of actiniclight. In the miniature lamp illustrated, the light-giving charge 10comprises a loose filling of filamentary material, such as shreddedfoil, of a readily combustible composition, such as aluminum, magnesiumor alloys thereof. The bulb 1 further contains a gas filling, such asoxygen for instance, at a suitable pressure for supporting combustion ofthe charge 10 of lightgiving material.

Illustrated in Fig. 2 is a flash lamp comprising a glass bulb 11 whichmay have a volume of approximately 25 cc. and is provided at its neckend 12 with a re-entrant stem 13 which extends into the bulb andterminates at its inner end in a press or seal portion 14 through whichis sealed a pair of lead-in wires 15, 16 connected at their outer endsto the center contact 17 and shell 18, respectively, of a base 19cemented or otherwise secured to the neck end of the bulb. The saidlead-in wires 15, 16 extend into the bulb in more or less side-by-siderelation and are provided at their inner ends with an ignition means 20substantially the same as that shown in Fig. l. The bulb 11 alsocontains a charge of readily combustible material 21 andcombustion-supporting gas as described above which, upon charge-flashingof the lamp, provides a momentary flash of actinic light.

Fig. 3 is a fragmentary sectional view of a flash lamp bulb showing theapplication of the protective coating of my invention thereto. Asillustrated in Fig. 3, the exterior bulb surface 23 is first coated witha layer of buffer lacquer 22. After the buffer lacquer has dried, theouter lacquer layer 24 is applied directly over the buffer layer.

According to the invention, the buffer layer comprises a thin film of asuitable material having efficient heat in sulating properties and beingtenacious with regard to acetone alcohol.

37 /z% by weight resin in about 62 /2% by weight acetone. Most of theacetone evaporates on drying of the lacquer leaving a thin, uniform,closely adherent film of isobutyl methacrylate which provides thedesired properties. The lamp bearing the buffer film is again immersed,as above, in a solution of outer lacquer. This lacquer is preferablycellulose acetate dissolved in acetone, alcohol and/or other suitablesolvents. For example, the preferred outer lacquer comprises about 15%by weight cellulose acetate dissolved in a mixture of solventscontaining about 80% by weight acetone, 10% by weight denatured alcohol,and 10% by weight di- Drying this outer lacquer provides a film ofcellulose acetate possessing the desired tensile strength.

In this manner the outer bulb surface of the lamp is provided with thedouble-layer protective coating of my invention comprising a bufferlayer adjacent the glass to absorb heat and the outer layer to provideadditional structural reinforcement.

. It will be noted that many materials exhibit the properties desired ina suitable buffer layer. polyvinyl resins, such as polyvinyl acetate,polyacrylates, and polymethacrylates, provide suitable heat insulatingfilms. Of the polymethacrylates I have found that ethyl, methyl, butyland isobutyl methacrylates and copolymcrs or mixtures thereof provideexcellent buffer films.

The most widely used outer lacquer in the photoflash art is celluloseacetate due to the fact that the material is so well adapted for thisparticular use and relatively inexpensive. Acetone is the principalsolvent for cellulose acetate, being very active and possessing a highevaporation rate. Suitable solvents possessing similar characteristics,such as ethylene dichloride or tricresyl phosphate for example, may besubstituted. However, any suitable material may be applied as an outerlayer which exhibits sufiicient tensile strength to structurallyreinforce the bulb during charge-flashing of the lamp. For exthe bulbsurface. It preferably should be of a material which softens oncharge-flashing of the lamp so that the lacquer becomes tacky preventingdispersion of glass particles should the bulb shatter. The material isdissolved in a suitable solvent having a fairly high evaporation rateproviding a solution which in its initial form issmooth flowing andadapted for application by dipping or spraying against the exterior ofthe bulb shell forming thereon a smooth rapid-drying coating of uniformthickness.

After the buffer coating is dry, the bulb is sprayed or dipped in asolution of asuitable material in a suitable solvent to provide theouter structurally reinforcing layer. A suitable outer layer material isa tearand abrasion-resistant plastic which in the dry solid stateexhibits a tensile strength of about 8000 to 10,000 p. s. i. and whichretains suficient strength during the short interval of elevatedtemperature, occasioned by flashing of the lamp, to effectivelyreinforce the glass bulb.

By way of specific example of an embodiment of my invention, acompletely assembled flash lamp is dipped in the buffer lacquer so thatthe bulb is completely immersed as well as a small portion of the basein the area of the base to bulb juncture. The lamp is then with drawnfrom the lacquer and dried. As a buffer lacquer I prefer to use asolution of isobutyl methacrylate in ace tone. A suitable compositionfor this lacquer is about ample, cellulose acetate-butyrate is amaterial suitable foruse as an outer coating possessing, as a dry film,the requisite strength and toughness. Combinations of cellulose acetateand cellulose acetate butyrate are also suitable for the desiredpurpose.

- The materials suitable for the buffer layer, as well as those suitablefor the outer lacquer film, may be dissolved in suitable solvents in anyproportion which will provide a solution of satisfactory viscosity sothat, in the method chosen to apply the coatings, smooth uniform filmsof the desired thickness will be obtained. The lacquer films should berelatively thin. For example, a suitable butfer film would be at leastabout 0.002 to 0.003 inchthick and the outer films at least about 0.003to 0.004 inch thick.

The two films may be applied by any suitable means such as dipping orspraying. Although a common solent may be used in both lacquers, thereis no bonding between the two films. I have found that the buffer andouter layers are completely separate and distinct. This feature may,partially at least, account for the particular efficiency with which thedouble lacquer coating performs the desired functions.

Since the lacquer films draw down tightly over the bulb on drying, andthe coefficients of thermal expansion of the lacquer materials and theglass ofthe bulb are fairly close, there is little, if any, diflicultyof separation of the lacquers from the bulb surface or from each other.Further, plastic films are generally somewhat elastic and intense heatis present for an extremely short period of time so there is littledanger of rupture of the filmsor their separation from the bulb, surfacedue to thermal expansion of either the glass orthe films.

I believe that the desired results are achieved by the buffer layerabsorbing most of the heat generated on flashing of the. lamp and alsodistributing the heat generated evenly over the entire bulb surface,thus prevent- For example,

ing the formation of hot spots which are concentrations of intense heatin a small area causing charring and often burning of the outer layer.The preferred methacrylate buffer materials, such as ethyl, methyl,butyl and isobutyl methacrylate, have a common property which partiallyaccounts, at least in theory, for their particular suitability as buiferfilms. These materials decompose with heat to monomers which completelyvolatilize at elevated temperatures rather than burn or char. Incontrast, most outer lacquer materials possessing the requisite strengthand other properties have been found to be those whose decompositionproducts are carbon or carbon compounds. Such products are combustibleand, at or above the decomposition temperature, will char or actuallyburn.

Other polyvinyl resins suitable for use in bufier layers may decomposeto monomers, although this has not definitely been established. In anyevent, these resins all absorb the heat generated on flashing of thelamp sufficiently to prevent excessive weakening or other damage to theouter layer. In addition, many of these bufier materials, although theymay not decompose to monomers, are heat-decomposable to products whichvolatilize almost completely at elevated temperatures and leave littleif any residue such as carbon or other residual material which mightdiscolor or char at the temperatures encountered. Any one or combinationof these properties enables the polyvinyl resins to effectively performthe hereinbefore described functions of a buffer layer.

The drying of the lacquer films on the bulb may be accelerated bypassing the bulbs through heated ovens or the films may be allowed todry naturally by evaporation of the volatile solvents. In addition,suitable well-known plasticizers may be added to the lacquers toincrease elasticity and plasticity of the films. However, in thespecific examples of lacquer compositions given, no plasticizer wasfound necessary.

Although a preferred embodiment of my invention has been disclosed, itwill be recognized that variations and changes may be made thereinwithin the spirit and scope of the invention as defined by the appendedclaims. It will be further understood that'the ingredients and theirproportions as given above can be varied, independently and in relationto each other, within fairly wide limits to obtain the desired results.Further, it will be particularly recognized that my invention is notlimited with respect to principal lacquer ingredients, good resultsbeing obtained with any lacquer or similar material possessing theproperties stated hereinbefore to be desirable in buffer and outercoatings.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A flash lamp comprising a sealed glass envelope, a quantity ofreadily combustible material and combustionsupporting means thereinadapted to generate on combustion a flash of actinic light, adouble-layer protective coating on the exterior surface of said envelopesaid coating comprising a butfer layer of light-transmittingheat-absorbing lacquer adjacent the exterior glass surface, andsuperimposed on said buffer layer an outer layer of structuralreinforcing light-transmitting lacquer possessing a dry tensile strengthof at least about 8000 p. s. i.

2. A flash lamp of the character set forth in claim 1 wherein the bufferlayer comprises essentially a polyvinyl resin.

3. A flash lamp of the character set forth in claim 1 wherein the bufferlayer comprises essentially a lacquer selected from the group consistingof polyvinyl acetate, ethyl methacrylate, methyl methacrylate, n-butylmethacrylate, isobutyl methacrylate and copolymers and mixtures thereof.

4. A flash lamp of the character set forth in claim 1 wherein the bufferlayer comprises essentially a polyvinyl resin and the outer layercomprises essentially at least one material of the group consisting ofcellulose acetate and cellulose acetate butyrate.

5. A flash lamp of the character set forth in claim 1 wherein the bufferlayer comprises essentially a lacquer selected from the group consistingof polyvinyl acetate, ethyl methacrylate, methyl methacrylate, butylmethacrylate, isobutyl methacrylate and copolymers and mixtures thereof,and the outer layer comprises essentially at least one material of thegroup consisting of cellulose acetate, cellulose acetate butyrate andmixtures thereof.

6. A flash lamp of the character set forth in claim 1 wherein the bufferlayer comprises essentially isobutyl methacrylate and the outer layercomprises essentially cellulose acetate.

References Cited in the file of this patent UNITED STATES PATENTS2,046,388 Kurlander July 7, 1936 2,277,776 Patton Mar. 31, 19422,324,959 Stevens July 20, 1943

1. A FLASH LAMP COMPRISING A SEALED GLASS ENVELOPE, A QUANTITY OFREADILY COMBUSTIBLE MATERIAL AND COMBUSTIONSUPPORTING MEANS THEREINADAPTED TO GENERATE ON COMBUSTION A FLASH OF ACTINIC LIGHT, ADOUBLE-LAYER PROTECTIVE COATING ON THE EXTERIOR SURFACE OF SAID ENVELOPESAID COATING COMPRISING A BUFFER LAYER OF LIGHT-TRANSMITTINGHEAT-ABSORBING LACQUER ADJACENT THE EXTERIOR GLASS SURFACE, ANDSUPERIMPOSED ON SAID BUFFER LAYER AN OUTER LAYER OF STRUCTURALREINFORCING LIGHT-TRANSMITTING LACQUER POSSESSING A DRY TENSILE STRENGTHOF A T LEAST ABOUT 8000 P.S.I.